Taking vitamin A with iron helps treat iron deficiency, since vitamin A improves the absorption and/or utilization of iron. This interaction has only been shown in a developing country with higher incidence of multiple deficiencies.

Refrences

Suharno D, West CE, Muhilal, et al. Supplementation with vitamin A and iron for nutritional anemia in pregnant women in West Java, Indonesia. Lancet 1993;342:1325-8.

Vitamin E appears to increase intestinal absorption and tissue uptake of vitamin A. Because of this effect, some doctors have used the combination of vitamin E and vitamin A for various skin conditions that are usually treated with vitamin A alone. The addition of vitamin E improved outcomes, while allowing for lower (and safer) doses of vitamin A.

Supplementation with zinc, iron, or the combination has been found to improve vitamin A status among children at high risk for deficiency of the three nutrients. This interaction has only been shown in a developing country with higher incidence of multiple deficiencies.

In one study, administration of 1.9 g per day of calcium for 6 weeks to healthy individuals reduced the uptake of iodine by the thyroid gland. Further studies suggested that this effect of calcium was due to inhibition of gastrointestinal absorption of iodine. To be on the safe side, some doctors recommend that people taking calcium for long periods of time should also take a multimineral supplement that contains iodine.

Vitamin C appears to convert sodium selenite (a form of selenium used for supplementation) to elemental selenium, making it unavailable for absorption. This interaction occurred when 1 g of vitamin C was taken with sodium selenite on an empty stomach, but not when these nutrients were taken together with a meal. Vitamin C does not appear to interact with selenomethionine, another commonly used form of supplemental selenium.

Refrences

Robinson MF,Thomson CD,Huemmer PK. Effect of a megadose of ascorbic acid, a meal and orange juice on the absorption of selenium as sodium selenite. The New Zealand medical journal. 1985 Aug;98(784):627-9

Ip C. Interaction of vitamin C and selenium supplementation in the modification of mammary carcinogenesis in rats. Journal of the National Cancer Institute. 1986 Jul;77(1):299-303

In a study of healthy men, supplementation with a large amount of zinc (142 mg per day) decreased magnesium absorption. Magnesium supplementation should therefore be considered for people taking large amounts of zinc, and possibly for those taking lower amounts.